The present invention relates to a process for operating an exhaust gas treatment unit for an internal combustion engine which is operated with lean air/fuel ratios over most of the operating period. The exhaust gas treatment unit contains a nitrogen oxides storage catalyst for converting the nitrogen oxides contained in the exhaust gas. To protect the storage catalyst from being poisoned by sulfur oxides, a sulfur trap to store sulfur oxides by producing sulfates is located upstream of the storage catalyst.
Nitrogen oxides catalysts were developed specifically for the exhaust gas treatment of lean-burn internal combustion engines. Lean operated internal petrol engines, so-called lean-burn engines, and diesel engines belong to the class of lean-mix operated engines. Lean-burn engines, in particular engines with direct injection of gasoline, are increasingly used in the construction of vehicles because they enable theoretical savings in fuel of up to 25% as compared with stoichiometrically operated internal combustion engines.
Carbon monoxide, unburnt hydrocarbons (HC), and nitrogen oxides (NO.sub.x) are the main harmful substances found in the exhaust gas of internal combustion engines. In addition, the exhaust gas also contains small proportions of hydrogen (H.sub.2) and sulfur oxides (SO.sub.x) which come from the sulfur content of the fuel and lubricating oil in the engine. A high percentage of the harmful substances, with the exception of sulfur oxides, can be converted into the harmless components water, carbon dioxide and nitrogen by modern exhaust gas catalysts, under stoichiometric operation. The catalysts developed for exhaust gas treatment of stoichiometrically operated internal combustion engines are known as three-way catalysts.
Stoichiometric conditions are present when the normalized air/fuel ratio .lambda. is 1. The normalized air/fuel ratio .lambda. is the air/fuel ratio normalized to stoichiometric conditions. The air/fuel ratio states how many kilograms of air are required for complete combustion of one kilogram of fuel. With conventional fuels, the stoichiometric air/fuel ratio has a value of 14.6. The air/fuel ratio of the exhaust gas emitted by an internal combustion engine corresponds to the air/fuel ratio of the air/fuel mixture supplied to the internal combustion engine. Exhaust gases with normalized air/fuel ratios of more than 1 are called lean, and exhaust gases with normalized air/fuel ratios of less than 1 are called rich.
Stoichiometric operation is maintained by controlling the air/fuel ratio supplied to the internal combustion engine. The signal from an oxygen sensor, a so-called .lambda.-sensor, is used for control purposes. Three-way catalysts can simultaneously convert the three harmful substances HC, CO and NO.sub.x in the exhaust gas only within a very narrow normalized air/fuel ratio interval between about 0.97 and about 1.03.
Whereas the purification of exhaust gases from stoichiometrically operated internal combustion engines has achieved a very high level, there are still considerable problems with conversion of nitrogen oxides emissions from lean operated internal combustion engines. During most of the operating period, these internal combustion engines have normalized air/fuel ratios greater than 1.3. The exhaust gas contains about 3 to 15% of oxygen. Thus, the conditions in the exhaust gas are strongly oxidizing. Under these conditions, the nitrogen oxides in the exhaust gas can no longer be reduced in a simple manner. The nitrogen oxides storage catalysts mentioned above, inter alia, have been developed to solve this problem.
The mode of operation and composition of nitrogen oxides storage catalysts are known for example from EP 0 560 991 B1. As storage material, these catalysts contain at least one component from the group of alkali metals (potassium, sodium, lithium, cesium), the alkaline earth metals (barium, calcium) or the rare earth metals (lanthanum, yttrium). The storage catalysts contain platinum as the catalytically active element. Under oxidizing exhaust gas conditions, that is under lean operation, the storage materials can store the nitrogen oxides contained in the exhaust gas in the form of nitrates. However, for this to occur, it is necessary that the nitrogen oxides, which contain about 50 to 90% of nitrogen monoxide depending on the construction of the engine and its mode of operation, are first oxidized to nitrogen dioxide. This takes place on the platinum component of the storage catalyst.
Since the storage capacity of a storage catalyst is restricted, it has to be regenerated from time to time. For this purpose, the normalized air/fuel ratio in the air/fuel mixture which is supplied to the engine, and thus also the normalized air/fuel ratio in the exhaust gas leaving the engine, is lowered to values of less than 1 for short periods. This is also called enriching the air/fuel mixture or exhaust gas. Thus, during these brief operating phases, reducing conditions prevail in the exhaust gas prior to its entrance into the storage catalyst.
Under the reducing conditions during the enrichment phase, the nitrogen oxides stored in the form of nitrates are released again (desorbed) and are reduced to nitrogen on the storage catalyst with simultaneous oxidation of carbon monoxide, hydrocarbons and hydrogen, as on a conventional three-way catalyst.
Despite their huge potential for removing nitrogen oxides from the exhaust gas of lean operated internal combustion engines, nitrogen oxides storage catalysts have not yet been used widely. One substantial problem when using nitrogen oxides storage catalysts is, in fact, the sulfur content of the fuel. This is emitted from internal combustion engines mainly in the form of sulfur dioxide. Sulfur dioxide acts as a catalyst poison in conventional three-way converter catalysts and in particular in nitrogen oxides storage catalysts. Poisoning by sulfur leads to a reduction in the conversion of harmful substances and to more rapid ageing of the catalyst in three-way catalysts. In general, the poisoning of three-way catalysts is largely reversible. The sulfur components in the exhaust gas are present in the form of sulfates on the three-way catalyst. Regeneration of the catalyst takes place under normal driving conditions during driving phases with high exhaust gas temperatures and a reducing exhaust gas. Under these conditions, the sulfates are reduced and the sulfur is emitted in the form of sulfur dioxide or hydrogen sulfide. The emission of hydrogen sulfide can be suppressed by specific measures relating to the catalyst and engine regulation.
Poisoning of a nitrogen oxides storage catalyst by sulfur oxides takes place in principle in the same way as the storage of nitrogen oxides. The sulfur dioxide emitted by the engine is oxidized to sulfur trioxide on the catalytically active noble metal component of the storage catalyst. Sulfur trioxide reacts with the storage materials in the storage catalyst with the formation of the corresponding sulfates. Particularly disadvantageous is the fact that the absorption of sulfur trioxide is preferred over the absorption of nitrogen oxides, and the sulfates which are formed are thermally very stable. Thus, there is a considerable reduction in the nitrogen oxides storage capacity of the catalyst as a result of poisoning by sulfur oxides which, in contrast to the situation in three-way catalysts, is reversible only at high exhaust gas temperatures, due to the high thermal stability of the sulfates of the storage materials, even under reducing exhaust gas conditions.
Strehlau et al (Conference "Motor und Umwelt" Graz, 1997, Proceedings, pages 15-30) found that sulfur can be removed from barium-containing storage catalysts to the optimum with exhaust gas temperatures just upstream of the catalyst of 650.degree. C. and with normalized air/fuel ratios of 0.98. These exhaust gas conditions can be set, even during part-load operation of vehicles by modifying the operating parameters of the engine. Modification of the operating parameters has to be performed in such a way that the least possible change to the torque is caused. Removal of sulfur at high exhaust gas temperatures, however, is associated with a considerable increase in consumption of fuel since the fuel is used simply to heat and condition the nitrogen oxides storage catalyst and is not converted into driving performance.
According to EP 0 582 917 A1, it has been suggested that the poisoning of a storage catalyst with sulfur can be reduced by a sulfur trap inserted into the exhaust gas stream upstream of the storage catalyst. Alkali metals (potassium, sodium, lithium and cesium), alkaline earth metals (barium and calcium) and rare earth metals (lanthanum and yttrium) are suggested as storage materials for the sulfur trap. The sulfur trap also contains platinum as a catalytically active component. However, the disadvantage of the proposals in EP 0 582 917 A1 is that removal of sulfur from the sulfur trap is not provided, that is to say that after reaching the full storage capacity of the sulfur trap, the sulfur oxides contained in the exhaust gas pass unhindered through the sulfur trap and can poison the downstream nitrogen oxides storage catalyst.
EP 0 625 633 A1 makes some improvement to this design. According to this document, a sulfur trap is also located in the exhaust gas stream of the internal combustion engine, just upstream of the nitrogen oxides storage catalyst. This combination of sulfur trap and nitrogen oxides storage catalyst is operated in such a way that sulfur oxides are stored on the sulfur trap and nitrogen oxides are stored on the nitrogen oxides storage catalyst under lean exhaust conditions. By periodically changing the exhaust gas conditions from lean to rich, the sulfates stored on the sulfur trap are decomposed to give sulfur dioxide and the nitrates stored on the nitrogen oxides storage catalyst are decomposed to give nitrogen dioxide. There is a risk here that sulfur dioxide and nitrogen dioxide react with each other over the nitrogen oxides storage catalyst to give sulfur trioxide and nitrogen monoxide and that sulfur trioxide is stored on the nitrogen oxides storage catalyst in the form of sulfates.
According to EP 0 625 633 A1, however, this type of reaction takes place to only a small extent since the rate of decomposition of nitrates is generally substantially higher than the corresponding rate of decomposition of sulfates. The decomposition of nitrates takes place in a short time interval of only about 5 to 20 seconds, while time intervals of up to 10 minutes are required for the complete decomposition of sulfates on the sulfur trap. Thus there is very little overlap of the times of emission of sulfur dioxide and nitrogen dioxide. Poisoning of the nitrogen oxides storage catalyst by sulfur during removal of sulfur from the sulfur trap can be kept low in this way. A further improvement is produced by highly enriching the exhaust gas to release nitrogen oxides from the nitrogen oxides catalyst and only slightly enriching the exhaust gas to release the sulfur oxides from the sulfur trap.
The quantities of sulfur oxides contained in the exhaust gas from an internal combustion engine are much smaller than the quantities of nitrogen oxides. Therefore, it is not necessary to also remove sulfur from the sulfur trap each time the nitrogen oxides are released from the storage catalyst. Whereas the period of the cycle for releasing nitrogen oxides from the nitrogen oxides catalyst is about one minute, the period of the cycle for releasing sulfur oxides from the sulfur trap is several hours, according to EP 0 582 917 A1.
The methods suggested so far for operating an exhaust gas treatment unit consisting of a sulfur trap and a nitrogen oxides storage catalyst have the disadvantage that deliberate poisoning of the nitrogen oxides storage catalyst is sometimes accepted.
An object of the present invention, therefore, is to provide an improved process for operating an exhaust gas treatment unit consisting of a sulfur trap and a nitrogen oxides storage catalyst which largely avoids the disadvantages mentioned above. In addition, there should be only a small increase in fuel consumption associated with using the desired process.